1. Field of the Invention
The invention relates to an image display device comprising an input for receiving information representing an image; drive means for generating a drive signal from the information; a cathode ray tube for displaying the image driven by the drive signal; and input means coupled to the drive means for receiving further information identifying a window of pixels in the image, the drive means being arranged to change a relation between the information and the drive signal in the window as compared to a further relation between the information and the drive signal in an area outside the window, so as to enhance a drive signal range in the window.
2. Description of the Related Art
Such a device is known from PCT Patent Application No. WO 96/17338, corresponding to U.S. Pat. No. 5,638,090. This device makes it possible to show an image which contains video and text and to enhance the luminosity range of the image in windows which contain video beyond the luminosity range of areas outside the windows which contain text. The increase in the luminosity range makes the video richer in contrast, but, on a CRT, it also increases the spot-size, which would be undesirable for text areas. By increasing the range of luminosity only locally where a window with video is shown, video with a richer contrast can be combined with text.
However, it has been found that by extending the luminosity range, one increases the risk of doming (deformation of the shadow mask due to heating by the electron beam) and overloading of the high voltage (EHT) power supply.